400hp 5.0

Well with that kind of budget a 347 is completely doable. If it were me I would use a TFS track heat intake along with a set of AFR heads, and a custom grind cam from Jay Allen. Very knowledgable man that can point out all the parts you will need to squeeze all the most available hp from that puppy.
 
Your an engine builder and you came to a forum to figure out how to make 400 hp on a SBF? Also, tell your customer unless he drives like a girl to skip the t-5 and get a TKO